(CNN) --An 8-year-old Florida boy left in critical condition after an apparent murder-suicide involving his father and three siblings died Tuesday, a hospital official said.

Bryan Randall was critically injured Monday morning when his father, Bryan Christopher Randall, drove his SUV into the path of a tractor-trailer on Interstate 4 in Lake Mary, near Orlando. (Full story)

The elder Randall died. Another son, Julian, 6, was injured in the crash.

Two other Randall children, 4-year-old Regal and 2-year-old Yana, were found in a pond Sunday in the Orlando suburb of Maitland. Regal, a boy, was barely alive and is in a hospital in critical condition. Yana, a girl, was dead.

Police believe the events were part of a murder-suicide plot by the father, who left behind a handwritten note in the front seat of the SUV.

"Dearest family and friends, 'Why?' must be the question on your minds, the following are the reasons, I made too many mistakes, I would not allow those beautiful children to grow up in the manner in which you had raised them, I didn't get the latest job. ..." Lake Mary police Chief Richard Beary read from the letter at a news conference Monday.

Beary said the note told authorities whom they should notify in case of the father's death, and indicated that the father wanted his and the childrens' bodies cremated and the ashes mixed together.

"It's just a terrible tragedy. You have a family that's been destroyed and the children, they're going to be devastated. This defies explanation. You can't rationalize this," Beary said.

The children's mother, Lisa Randall, 41, told authorities she had left the children with their father Friday and that he was to return them Sunday.

The mother and father had been married for 10 years but were getting a divorce.

CNN correspondent Susan Candiotti in Miami contributed to this report.